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Emulsion-induced ordered microporous films using amphiphilic poly(ethylene oxide)-block-poly(n-butyl isocyanate) block copolymers.

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Emulsion-Induced Ordered Microporous Films Using
Amphiphilic Poly(ethylene oxide)-block-Poly(n-butyl
isocyanate) Block Copolymers
Koji Ishizu, Masataka Makino, Naomasa Hatoyama, Satoshi Uchida
Department of Organic Materials and Macromolecules, International Research Center of Macromolecular Science,
Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received 29 October 2007; accepted 4 December 2007
DOI 10.1002/app.28030
Published online 12 March 2008 in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Well-defined poly(ethylene oxide)-blockpoly(n-butyl isocyanate) (PEO-block-PBIC; AB) diblock
copolymers were prepared with trichloropentadienyl titanium (IV) (CpTiCl3) catalyst. Ordered microporous films
(hexagonal pattern) were constructed by emulsion
micelles of A110B230 diblock copolymer (the subscript
indicates the degree of polymerization of each block
component) formed from CHCl3/H2O/tetrahydrofuran
(THF) 5 100/5/10 (v/v) solution. The addition of THF
promoted uniform micelles in immiscible CHCl3/H2O
mixed solvent as well as the effect of phase transfer catalyst. Diblock copolymers formed spherical micelles, consisting of a PEO core and a PBIC corona. At some con-
centration, interaction between micelles forced a disorder–order transition, resulting in freezing of micelles in
some superlattice. Subsequently, the leaving gaps among
micelles were filled by PBIC block chains on a corona
through the evaporation of organic solvents. At last,
water within hydrophilic PEO core was vomited through
the film matrix. We also constructed the microporous
films using A110B230 diblock copolymer by the current
water-assisted method. Ó 2008 Wiley Periodicals, Inc. J Appl
INTRODUCTION
spreading method and on-water spreading method
based on self-organization of water droplets. Moreover, Imhof and Pine8 reported the microporous
structure to use sol-gel processing to deposit an inorganic material at the exterior of the droplets in a
monodisperse emulsion.
More recently, we have provided a new route to
construct the ordered microporous films based on Y
solvent-induced mechanism using core-shell nanospheres9 and diblock copolymers.10 According to our
results, core-shell nanospheres or core-corona
micelles (formed by diblock copolymers) formed the
lattice of body-centered cubic (bcc) structure near
the overlap threshold (C*) by temperature control of
casting solvent. The shell or corona parts were composed of polystyrene (PS) segments. Therefore, leaving gaps among spherical lattices were filled with
cyclohexane solvent (Y solvent for PS). After evaporation of the solvent, polymer films having ordered
micropore pattern were formed.
On the other hand, polyisocyanates (PIC) are unusual class of polymeric materials that adopt helical
conformations both in solution and in bulk.11–14
Numerous characterization techniques have shown
that PIC is stiff-chain polymers whose properties
depend on several parameters, such as the nature of
the isocyanate side group, temperature, solvent, and
molecular weight.15,16 Consequently, they may behave
Microporous materials, those with pore diameters
greater than 50 nm, have a wide range of application
in chemistry. Since François and coworkers1 first utilized the condensation of monodisperse water droplets on polymer solution to fabricate honeycomb
films with monodispersed pores, in which pores
exhibited as hexagonal arrays, their method aroused
great interest. A variety of polymers, such as star
and comb polymers, and block copolymers were
used to obtain honeycomb-structured films,2–4 and
the concept of such water-assisted patterning was
extended. Especially, Stenzel and coworkers have
reported the preparation of microporous films of
block copolymer micelles5 and comb-like polymers,6
using the breath figures templating technique. On
the other hand, dendronized polymers are rod-like
in shape due to the steric hindrance imposed by the
bulky dendritic side groups attached to each repeating unit. Cheng et al.7 have reported the fabrication
of honeycomb-patterned films from the amphiphilic
dendronized block copolymers by on-solid surface
Correspondence to: K. Ishizu (kishizu@polymer.titech.
ac.jp).
Journal of Applied Polymer Science, Vol. 108, 3753–3759 (2008)
C 2008 Wiley Periodicals, Inc.
V
Polym Sci 108: 3753–3759, 2008
Key words: rod-coil diblock copolymer; microporous film;
emulsion-induced method; water-assisted method
3754
ISHIZU ET AL.
either as rigid rods or as semiflexible, wormlike chains.
A combination of PIC with flexible chains in diblock
copolymers or more complex macromolecular architectures provides new challenges in nanotechnology
regarding the microphase separation, the ordering
kinetics, and the self-assembly behavior in solution.17–19
In this article, well-defined poly(ethylene oxide)block-poly(n-butyl isocyanate) (PEO-block-PBIC) diblock copolymers were prepared with trichlorocyclopentadienyl-titanium (IV) (CpTiCl3) catalyst. It is discovered that ordered microporous films are constructed by emulsion micelles of such amphiphilic
diblock copolymers formed from chloroform
(CHCl3)/water (H2O)/tetrahydrofuran (THF) solution or by the current water-assisted method. The
formation mechanisms of both microporous films are
discussed in detail.
EXPERIMENTAL
Materials
n-Butyl isocyanate (BIC; Tokyo Kasei Organic Chemicals, Tokyo) was dried over calcium hydride (CaH2)
and distilled in vacuo. Poly(ethylene glycol) methyl
ether (PEO; Mn 5 2000 and 5000), CpTiCl3 (Aldrich,
Milwaukee), CHCl3, THF, diethyl ether, acetic anhydride (Tokyo Kasei Organic Chemicals, Tokyo), and
CaH2 (Kanto Kagaku Reagent Division, Tokyo) were
used as received.
Synthesis of PEO-block-PBIC diblock copolymers
The living coordination polymerization of isocyanates was reported by Patten and Novak20,21 using
organotitanium (IV) complexes of the type
TiCl3(OCH2CF3) and CpTiCl2L [Cp 5 cyclopentadiene and L 5 OCH2CF3, N(CH3)2, or CH3].
The replacement of one of the chlorine atoms with
the bulkier and more electron-donating Cp group
reduces the Lewis acidity of the titanium center, and
the polymerization of isocyanate proceeds in a
slower but more controlled manner. With this methodology, several isocyanates were polymerized. Furthermore, a diblock copolymer of PEO and poly(nhexyl isocyanate) was synthesized.22 Then, PEOblock-PBIC diblock copolymers were prepared in accordance with above reference. The synthesis route
is shown in Scheme 1. All procedures were carried
out with standard high-vacuum techniques using
break-sealed method. CpTiCl2(OPEO) macromolecular catalyst was prepared from a mixture of CpTiCl3
and PEO (1 : 1 molar ratio) in THF (dried over
CaH2) at 408C and the solvent was removed in vacuo.
This macromolecular catalyst was dissolved in BIC.
The polymerization was allowed to take place at
room temperature for 24 h under rigorous stirring.
Journal of Applied Polymer Science DOI 10.1002/app
Scheme 1
The viscosity built up rapidly, and the solution solidified. Termination was achieved by the addition of
acetic anhydride. THF was added to dissolve the
polymer. The polymer was precipitated into diethyl
ether and dried in a vacuum oven.
Preparation of microporous films
Microporous films were constructed by following
two methods. (a) Emulsion-induced method: A typical procedure was as follows. The emulsion solution
of amphiphilic diblock copolymers [0.1 wt % solution of CHCl3/H2O/THF 5 100/5/10 (v/v)] was
prepared under ultrasonic irradiation and this solution was cast on the mica substrate, where the solvent was evaporated as slowly as possible at 208C.
(b) Water-assisted method: a 0.1 wt % CHCl3 solution of diblock copolymers was cast on the mica substrate. The vapor of water (produced by bubbling of
N2) was applied to the solution surface for a few
minutes. Subsequently, the solvent was evaporated
as slowly as possible at 208C.
Measurements
The polydispersity (Mw/Mn) of diblock copolymers
was determined by gel permeation chromatography
(GPC; Tosoh HLC-8020 high-speed liquid chromatograph, Tokyo) with CHCl3 as eluent at 308C, two
TSK gel columns, GMHXL and G2000HXL, in series,
and a flow rate of 1.0 mL/min using PS standard
samples as calibration.
The composition of PEO-block-PBIC was determined by 1H-NMR (500 MHz, JEOL GSX-500 NMR
spectrometer, Tokyo) in CDCl3. The spectra exhibited the expected resonances assignable to methyl
protons (d 0.94 ppm) of n-butyl groups of PBIC and
methylene protons (3.65 ppm) of PEO. The number–
average molecular weight (Mn) of diblock copolymer
EMULSION-INDUCED ORDERED MICROPOROUS FILMS
3755
TABLE I
Polymerization Conditions and Results of Diblock Copolymersa
Polymerization conditions
b
c
Diblock copolymers
Code
102[I] (mmol)
[M] (mmol)
Temp. (8C)
A45B145
A110B230
15.3
23.0
8.88
44.4
25
40
24
10
Mn
d
1.64
2.78
Mw/Mne
PBIC blockf (mol %)
1.51
1.77
76.3
67.6
a
Polymerized in bulk for 24 h.
Macromolecular catalyst CpTiCl2(OPEO).
c
Monomer n-butyl isocyanate.
d
Determined from Mn of PEO precursor and composition of diblock copolymer.
e
Determined by GPC profiles in CHCl3 as eluent using PS standard samples.
f
Determined 1H NMR spectrum in CDCl3.
b
was evaluated from the Mn of PEO precursor and
composition of diblock copolymer.
The hydrodynamic radius (Rh) of emulsion
micelles formed by diblock copolymers was evaluated using Stokes-Einstein equation from the diffusion coefficient (D0) determined by dynamic light
scattering (DLS; Photal TMLS-6000HL: Otsuka Electronics, Tokyo, He-Ne laser: k0 5 632.8 nm) data
with cumulant method in the prescribed solution at
258C (scattering angle of 908). Mixed solvent was filtered through membrane filters with a nominal pore
of 0.2 lm just before measurement. Emulsion
micelles of diblock copolymers were prepared by
using this solvent under ultrasonic irradiation.
The morphology and pore size (Dn) of microporous films were investigated by the use of a JEOL
JSM-T220 (Tokyo) scanning electron microscope
(SEM) with a tilt angle of 308. To observe the vertical
section of microporous films, the film was broken in
liquid nitrogen. The specimen was sputtered with
gold. The pore size distribution (Dw/Dn) was determined by a survey of 300 samples picked up from
the photographs that were obtained. Dn and Dw are
defined as follows:
Dn ¼
Dw ¼
X
X
ni Di =
X
ni D2i =
Typical GPC profiles of diblock copolymer
A110B230 and PEO precursor are shown in Figure 1
using refractive index (RI) detector, where the subscripts indicate the degree of polymerization (DPn)
of each block component. The GPC distribution of
A110B230 has a single and relatively broad polydispersity (Mw/Mn 5 1.77) and shifts to the high-molecular-weight side compared with that of the PEO precursor. To elucidate the Mn of diblock copolymer,
we determined the composition of diblock copolymers by 1H-NMR spectra in CDCl3. Figure 2 shows
a typical 1H-NMR spectrum of A110B230 diblock copolymer. The single peak observed at d 3.65 ppm (a)
is attributed to the methylene protons of PEO block.
A strong peak at 0.94 ppm (e) is assignable to the
methyl protons of n-butyl group of PBIC block. The
composition of PBIC block (67.6 mol %) was determined from the signal intensity ratio of methyl protons
(e) to the methylene protons (a). Subsequently, the Mn
of diblock copolymer was evaluated from the Mn of
PEO precursor and composition of diblock copolymer.
These characteristics are also listed in Table I.
Water-assisted microporous films
ni
X
ni Di
We first employed the current water-assisted method
for the fabrication of microporopus films. Prelimi-
where Di (i 5 1,2, . . . ,q) and ni are pore size and
mole fraction, respectively.
RESULTS AND DISCUSSION
Synthesis and characterization of
diblock copolymers
Well-defined PEO-block-PBIC diblock copolymers
(AB: A and B indicate PEO and PBIC blocks, respectively) were prepared by living coordination polymerization. The polymerization conditions and
results of diblock copolymers are listed in Table I.
Figure 1 GPC profiles of A110B230 diblock copolymer and
PEO precursor.
Journal of Applied Polymer Science DOI 10.1002/app
3756
ISHIZU ET AL.
The mechanism of formation of the pore morphology can be speculated as follows (see the illustration
in Fig. 5). Solvent evaporation increases the superficial concentration and induces a cooling of the solution surface. Hydrophilic PEO chains stop the water
droplets by throwing their arms and stabilize sterically water droplets on the surface of casting solution. After CHCl3 solvent evaporation, hydrophobic
PBIC block chains form a continuous matrix. At last,
water droplets are evaporated and a polymer film
that has the ordered micropore pattern at the film
surface is formed.
Figure 2 1H-NMR spectrum of A110B230 diblock copolymer in CDCl3.
nary experiments were carried out using A45B145
diblock copolymer, varying the concentration of casting solution (0.05–1.0 wt % CHCl3 solution). As a
result, 0.1 wt % casting solution indicated moderate
micropore patterning as shown in Figure 3 (top view
of SEM photograph; Dn 5 60–400 nm). Then, the
film fabrication was performed using A110B230
diblock copolymer composed of long block segments
under the same condition. Typical SEM photographs
of film specimens are shown in Figure 4, where
Figures 4(a–c) indicate the top view, the vertical section, and pore size distribution, respectively. It is
found from Figures 4(a,c) that highly ordered micropores (average-pore size Dn 5 1300 nm) are clearly
visible for the sample surface but the pore size distribution (Dw/Dn 5 1.21) is relatively broad. The vertical section as shown in Figure 4(b) also indicates
that such microporous film is constructed with single layer. The pore sizes decreased with increment
of Mn of each block sequence as the template.
Figure 3 Top view of SEM photograph of microporous
film A45B145 prepared by water-assisted method.
Journal of Applied Polymer Science DOI 10.1002/app
Emulsion-induced microporous films
PEO-block-PBIC diblock copolymers can be speculated to form emulsion micelles due to amphiphilic
nature. First, the emulsion solution of A110B230
diblock copolymer [CHCl3/H2O 5 100/5 (v/v)] was
prepared under ultrasonic irradiation and this solution was cast on the mica substrate, where the solvent evaporated as slowly as possible. Figure 6(a)
shows the top view of SEM photograph of the film
specimen. The micropores (Dn 5 640 nm) are clearly
visible for the sample surface but the pore size distribution seems not so narrow. This means that
Figure 4 Top view (a) and vertical section (b) of SEM
photographs, and pore size distribution of microporous
film A110B230 prepared by water-assisted method.
EMULSION-INDUCED ORDERED MICROPOROUS FILMS
3757
the rearrangement of emulsion micelles occurs during solvent evaporation, because ternary solvent
composition changes in the process of solvent evaporation. A problem concerning the kinetics of how the
micelles developed when the solvents evaporated
has been left unsolved.
The emulsion-induced mechanism (see the schematic illustration in Fig. 8) of formation of highly ordered pore morphology can be speculated as follows. The PEO-block-PBIC diblock copolymers form
spherical micelles, consisting of a PEO core and a
PBIC corona, because CHCl3 fraction is too high. At
some concentration, interaction between micelles
will force a disorder–order transition, resulting in
freezing of micelles in some superlattice. In a nonselective solvent of diblock copolymers, this corresponds to a body-centered cubic (bcc) superstructure.23 Subsequently, the leaving gaps among
micelles are filled by PBIC block chains on a corona
through the evaporation of organic solvents such as
CHCl3 and THF. At last, water within hydrophilic
PEO core is vomited through the film matrix. PEO
phase also deforms to hexagonal shape. Thus deformation behaviors were also observed in latex film24
Figure 5 Schematic illustration of a microporous film of
diblock copolymer prepared by water-assisted method.
emulsion micelles formed are not so uniform. Figure 6(b) shows the micelle size distribution on DLS
data of corresponding emulsion solution of A110B230
at 258C. This profile has unimodal distribution (particle diameter Dh 5 780 nm). The pore size of the film
obtained is in well agreement with such micelle size.
Next, we examined the effect of organic/aqueous
solvent phases on the formation of emulsion
micelles. The emulsion solution of A110B230 diblock
copolymer was prepared in CHCl3/H2O/THF [100/
5/10 (v/v)] ternary system. Figure 7(a) shows the
top view of SEM photograph for the film specimen.
It is found from this photograph that highly ordered
hexagonal microspheres are clearly visible for the
sample surface. Figure 7(b) shows pore size distribution of this specimen. The average pore size (Dn) is
estimated to be 850 nm and the pore size distribution (Dw/Dn 5 1.10) is very narrow. THF is miscible
with both CHCl3 and water. Therefore, the addition
of THF was speculated to promote uniform micelles
in immiscible CHCl3/H2O mixed solvent as well as
the effect of phase transfer catalyst. Figure 7(c)
shows the micelle size distribution on DLS data of
corresponding solution of A110B230 at 258C. This profile has unimodal distribution (Dh 5 2200 nm). However, the micelle size is extremely larger than the
pore size obtained (Dn 5 850 nm). As mentioned
earlier, the average micelle size was 780 nm, when
CHCl3/H2O was used [see Fig. 6(b)]. It seems that
Figure 6 Top view of SEM photograph of microporous
film A110B230 prepared by emulsion micelles [CHCl3/H2O
5 100/5 (v/v)] (a) and size distribution on DLS data in
0.1 wt % CHCl3/H2O [100/5 (v/v)] solution at 258C.
Journal of Applied Polymer Science DOI 10.1002/app
3758
ISHIZU ET AL.
and crosslinked core-shell nanospheres.25 An important question of why the leaving gaps are filled with
rod PBIC blocks has been left. In general, rod-coil
diblock copolymers such as poly(n-hexyl isocyanate)-block-PS showed a zig-zag lamellar morphology.18 In fact, the cast film (CHCl3 solvent) of
A110B230 diblock copolymers used in this work also
Figure 8 Schematic illustration of a microporous film of
diblock copolymer by emulsion-induced method.
showed a zig-zag-like lamellar morphology. PBIC
may not behave as the rigid polymer, because
this polyisocyanate takes dynamic helical conformation in solution. Then, the generation of microporous
films by emulsion-induced method seems universal
phenomena for amphiphilic coil–coil diblock
copolymers.
CONCLUSIONS
Ordered microporous films are constructed using
PEO-block-PBIC amphiphilic diblock copolymers by
emulsion-induced and water-assisted methods. Especially, emulsion-induced method is a new route to
construct the ordered microporous films based on
the superlattice formation of emulsion micelles. The
pore sizes may be possibly controlled by micelle
sizes, i.e., Mn of diblock copolymers. This work
raises the possibility that such microporous structures would be formed by general coil type of
amphiphilic diblock copolymers.
References
Figure 7 Top view of SEM photograph (a), pore size distribution (b) of microporous film A110B230 prepared by
emulsion micelles [CHCl3/H2O/THF 5 100/5/10 (v/v)],
and size distribution on DLS data in 0.1 wt % CHCl3/
H2O/THF [100/5/10 (v/v)] solution at 258C.
Journal of Applied Polymer Science DOI 10.1002/app
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Journal of Applied Polymer Science DOI 10.1002/app
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